Storable composition and slurries and use of same for cementing oil and gas wells

ABSTRACT

Storable compositions and slurries for cementing oil and gas wells are described which comprise blast-furnace slag besides other additives which are usual in the cementing technique. The compositions and slurries are insufficiently activated (or are activated immediately before introducing them into the well), so that they can be stored for at least 72 hours (alternatively they may be stored indefinitely) without the setting of the composition and dispense with the addition of activation inhibitors. The use of the compositions and slurries in the cementing of oil and gas wells is also described.

FIELD OF THE INVENTION

[0001] The present invention relates to compositions and slurries forcementing oil and gas wells as well as to the use of such slurries inthose wells, and specifically to compositions and slurries containingblast furnace slag to be used in water-based fluids as hydraulicmaterial, in the absence of set inhibitors, such slurries being storableat ambient temperature.

BACKGROUND INFORMATION

[0002] The usual procedure for drilling an oil and gas well comprisesdrilling the well using a drilling fluid. Then, a casing is introducedin the well and a cementitious slurry is placed in the annulus betweenthe outer portion of the casing and the borehole wall. The purpose ofthe cementitious slurry is to harden and provide efficient sealingbetween the casing and the well. The outer shell of hardened cementimpedes or inhibits the fluid communication among the various formationspenetrated by the well. Besides providing isolation among zones, thecement contacts the casing and provides mechanical support and strengthto axial movement through the adherence developed between the interfacesof cement and casing as well as between the interfaces of the cement andthe formation.

[0003] Besides the proper features of adherence between the cement andthe casing and between the cement and the formation, of compatibilitybetween the cementitious slurry and the drilling fluid, of thecompressive strength and of the Theological parameters, anotherimportant feature is the set time (or hardening time) of the slurrycompositions. Normally, slurries contain additives for the control ofset time (activators and retarders) and rheology-modifying additives.

[0004] The set activating additives are used in the cementitiousslurries aiming at avoiding long waiting times (up to ca. 24 hours)until the slurry hardens, after the slurry being placed in the desiredsite of the annulus of the well.

[0005] The patent literature is abundant in publications dealing withapplications of blast furnace slag together with water based drillingfluids.

[0006] Thus, U.S. Pat. No. 5,058,679 teaches the in situ solidificationof water based fluids by adding blast furnace slag, additives forcontrolling the set time, as well as rheology modifiers. After additionfor example 50 to 400 lb/bbl slag, the drilling fluid is displaced tothe annulus and left to harden. The setting time of the drilling fluidtreated with slag may be varied according to the temperature conditionsof the well and the requirements for the displacing time. Set times arecontrolled between around one hour and up to several days fortemperatures between 40 and 400° F. (4 and 204° C.).

[0007] U.S. Pat. No. 5,409,064 teaches a composition and method forcementing a well by combining ingredients which comprise water and blastfurnace slag of particle size between 2,000 cm²/g and 15,000 cm²/g (200m²/kg and 1,500 m²/kg), preferably between 4,000 cm²/g and 6,500 cm²/g(400 m²/kg and 650 m²/kg) and an activator which comprises sodiumpyrophosphate to form a cementitious slurry; displacing the cementslurry to the well, and allow the cementitious slurry to set.

[0008] U.S. Pat. No. 5,447,197 teaches storable cementitious slurriesmade from cementing hydraulically active material, which may be storedin the liquid state. The slurry is activated when needed for cementing.After pumping the activated slurry to the interior of the well, theproperties of the hardened cement are suitable for well cementing. Thecomposition contains a set retarder aiming at maintaining the slurry inthe liquid state while stored, and an activator to revert theretardation caused by the retarder.

[0009] U.S. Pat. No. 5,547,506 teaches liquid storable cementitiousslurries made from hydraulically active cementitious material, a setinhibitor to allow storage of the slurry and an activator to activatethe slurry.

[0010] A paper published by the Society of the Petroleum Engineers (SPE)no. 36973, of October 1996, teaches a new technology for cementing ofslim wells with the aid of Liquid Cement Premix (LCP), such technologyhaving the advantage of providing slurries having a more uniform densitywhile dispensing with the use of expensive mixers in the field.

[0011] Conceptually, LCP is a storable cement slurry which may beindefinitely kept in the liquid state so that the set is determined asrequired. The slurry is a pre-mix of Portland cement or any otherhydraulic material containing set inhibitors and other additives. Suchsystem requires the use of an activator, which is added immediatelybefore pumping the slurry to the well. The activator resumes hydrationand allows that the slurry sets and develops compressive strength. TheLCP properties may be adjusted with the aid of the additives used inconventional cementing operations. The slurry is compatible with all theusual cementing additives. LCP attains the usual densities. Values ofcompressive strength developed after around 3 hours are suitable forwell cementing. The range of useful temperatures extends from ambienttemperatures to 450° F. (230° C.).

[0012] Thus, the literature teaches cementing slurries which use waterbased drilling fluids and blast furnace slag as hydraulic material, theslurries being pumped to the well soon after being made. The cementingslurries of the state-of-the-art technique necessarily comprise the useof retarders/activators, this representing the presence of more reagentsin the composition, besides higher cost. So, the technique is still inneed of compositions and storable cementitious slurries for wellcementing which may be stored at ambient temperature, dispensing withthe use of inhibitors/activators, such compositions and slurries beingprovided for by the present invention.

SUMMARY OF THE INVENTION

[0013] The storable compositions of cementitious slurries for oil andgas wells of the present invention are based on the presence of blastfurnace slag as the hydraulic material as well as on specific amounts ofother components of the composition, the compositions and slurries beingfree of set retarders.

[0014] The storable compositions of cementitious slurries according tothe present invention comprise:

[0015] Of from 200 to 500 lb of blast furnace slag/bbl of slurry (570 to1428 kg of blast furnace slag/m³ of slurry);

[0016] Of from 0.01 to 13 weight % of activators, based on the weight ofblast furnace slag;

[0017] The balance being made up of water based fluid,

[0018] The composition may still contain other usual additives forcementing and drilling fluid for oil and gas wells.

[0019] Contrary to the storable compositions of cementitious slurry ofthe state-of-the-art technique, the storable compositions of the presentinvention are formulated in the absence of any amount of set retarders.

[0020] In terms of concentration, the compositions are defined as: blastfurnace slag, kg/m³, the activators, weight % based on the weight ofblast furnace slag and the possible additives, weight % based on theweight of blast furnace slag or kg/m³.

[0021] Water or a water-based drilling fluid or a mixture of sameconstitute the continuous medium.

[0022] Another composition of cementitious slurry according to theinvention contains zero percent activators at the time of thepreparation, the activators being added immediately before pumping theslurry to the wellbore.

[0023] Still another composition of cementitious slurry according to theinvention contains a portion of the activators at the moment it isprepared the remaining of the activators being added immediately beforepumping the slurry to the wellbore.

[0024] In view of the three parameters which control the setting time ofthe slag-containing cementitious slurries:

[0025] physical and chemical properties (surface area, vitrificationdegree and chemical composition) and concentration of the blast furnaceslag;

[0026] kind and concentration of chemical activators;

[0027] wellbore temperature,

[0028] The principle on which are based the compositions of theinvention storable at ambient temperature in the absence of setretarders relates to the control of the blast furnace slag and thechemical activators.

[0029] It should be understood that the temperature of the wellborewhere the slurries will be utilized varies within broad limits, and thatsuch parameter cannot be controlled.

[0030] Still under another point of view, the compositions and storableslurries of the present invention may be considered as insufficientlyactivated, so that the set does not occur at the surface (out of thewell). Activation will only occur when the compositions and cementitiousslurries are chemically and thermally activated this occurring throughaddition of activators and by the inner temperature of the wellbore.Thus, without the need of the set retarders of the state-of-the-art,liquid slurries are obtained which are storable at ambient temperaturesduring prolonged periods, for example, 72 hours or more.

[0031] Thus, the present invention provides compositions andcementitious slurries storable at ambient temperature to be utilized inoil and gas wells, vertical, deviated and horizontal wells, saidslurries being formulated based on blast furnace slag as hydraulicmaterial, activators, water-based drilling fluid or water, or a mixtureof same, as well as usual additives for drilling fluid and cementingslurries.

[0032] The present invention provides further compositions andcementitious slurries storable at ambient temperature where the slurryis completely free of activator, the activator being added immediatelybefore the slurry is pumped to the wellbore.

[0033] The present invention provides still compositions andcementitious slurries storable at ambient temperature where the slurrycontains a portion of the activator required for the set while anotherportion of the activator is added immediately before the slurry ispumped to the well.

[0034] The present invention provides further compositions andcementitious slurries storable at ambient temperature where the properamount of the activation parameters allows that the slurry be kept inthe liquid state for prolonged periods of 72 hours or more, dispensingwith retarders specifically designed to retard the setting of the slurrycompositions as well as activators for reversing the effect of theretarder.

[0035] Still, the present invention provides an improvement in theoperation of oil and gas well cementing through the homogeneity of theslag-containing slurry, which is obtained without the risk of setting atthe surface, as well as by the simplicity in the operation, thisyielding less risk of operation failure and lower operation period.

DETAILED DESCRIPTION OF THE PREFERRED MODES

[0036] The preferred modes described below together with the attachedExamples illustrate the principles of the invention.

[0037] For the purposes of the present specification and claims, setretarder, or retarder, is every additive utilized in the slurry aimingat preventing the hardening or setting of the slurry before said slurryis pumped to the well annulus, and set inhibitor or inhibitor is everyadditive utilized in the slurry aiming at allowing the storage of samein the liquid state for prolonged periods, for example 72 hours or more.

[0038] Advantageously, the present invention provides compositions andstorable slurries aiming at the well cementing which may be prepared inadvance, stored, moved and taken to the site of utilization at the mostconvenient time, without the need of immediate use common to thestate-of-the-art slurries. Such advantage is obtained exclusively byadjusting the relative amounts of the slurry constituents, dispensingwith set inhibitors and activators to revert the action of theinhibitors, such as is well known for the conventional slurries.

[0039] The state-of-the-art cementing operation requires that the slurryis displaced to the wellbore as soon as the hydraulic material is mixedto the mixing water (water plus additives), in order to prevent that theset occurs in the tanks and cementing lines or still within the casing.Any of these situations constitute an operation drawback, besidesdamages for loss of material, working hours and waste of rig time, andeven ultimately loss of the well. This way of conducting the cementingoperation, by mixing hydraulic material and mixing water, followed bydisplacement to the wellbore causes a variation in the concentration ofhydraulic material in the slurry, this in turn entraining a variation inphysical properties (density, compressive strength, etc.) which possiblyleads to low quality cementing to be corrected by squeezing, with theconsequent increased costs.

[0040] One way to avoid the oscillation in the concentration ofhydraulic material is to utilize a mixer able to prepare the entirerequired amount for the cementing operation in one single batch (aso-called batch-mixer). However the slurry prepared using this type ofmixer increases the cost of the cementing operation of oil and gaswells.

[0041] Typically, cementitious slurries comprise cement, fresh or saltedwater and various other additives such as extenders, strengthretrogression controllers, accelerators and retarders, fluid lossadditives, dispersants and weighting agents.

[0042] In order that the cementitious slurries show the desiredproperties, various additives are being used. Non-limiting examples arebentonite, silica, calcium chloride, lignosulfonate and relatedproducts, boric acid, hydroxy ethyl cellulose, carboxy methyl celluloseand other cellulose derivatives, tartaric acid, citric acid and othercarboxylic acids, naphthalene sulfonic acid salt, barite and hematite.

[0043] In the present specification and claims, the term “water basefluid” means a fluid wherein the continuous phase is water. Non-limitingexamples are fresh water, brine, sea water or other aqueous fluidsisolated or combined, such as water-based drilling fluids.

[0044] Water-based drilling fluids typically comprise water and variousother additives such as viscosifiers, fluid loss additives, dispersants,dissolved salts, alkaline agents, solids from the perforated formation,weighting agents, formation stabilizers, lubricants for the drillingfluid and anti-foaming agents.

[0045] In order that the drilling fluids show the desired properties,various additives have been used. Non-limiting examples are bentonite,polyacrilamide, guar gum, xanthan gum, carboxy methyl cellulose, starch,tannin compounds, lignosulfonate, polyacrylate, sodium chloride,potassium chloride, calcium chloride, sodium hydroxide, potassiumhydroxide, calcium hydroxide, clays, barium sulfate or barite, ironoxide, calcium carbonate, lubricating oil, octylic alcohol and aluminumstearate

[0046] The solidification of a drilling fluid may be obtained by addingblast furnace slag as hydraulic material together with additives for thecontrol of set time and rheological properties of the fluid, which hasbeen slag-treated.

[0047] Blast furnace slag is a non-metallic product, which consistsessentially of calcium silicates and calcium aluminosilicates, and otherbases, and is produced in the melt state simultaneously with iron in ablast-furnace. Blast-furnace slag is therefore a by-product of themanufacture of iron and results from the combination of minerals fromthe residuum of the iron ore, the coke ashes and from the lime used assmelter. As the density of slag is lower than that of iron, the slag ispoured in the melt as the upper phase at temperatures usually between2250° C. and 2900° C. Such mixture is then quickly cooled aiming atobtaining glassy and powdery material, able to develop hydraulicactivity.

[0048] Blast furnace slag (or slag) shows the same compounds as cement,however, at different concentrations. TABLE 1 below illustrates thetypical chemical composition of blast furnace slag and Portland CementClass G, both Brazilian made. TABLE 1 BLAST FURNACE SLAG PORTLAND CEMENTCOMPOSITION (WEIGHT %) Class G (weight %) CaO 33 to 44 48 to 52 SiO₂ 32to 44 26 to 29 Al₂O₃ 12 to 20  7 to 11 MgO 1 to 7 4 to 6 Fe₂O₃ 0 to 2 2to 3 FeO 0 to 2 — K₂O 0 to 2 <1   Na₂O   <0.5 <0.3 TiO₂ <1 — Mn₂O₃ <1 —SO₃   <0.1 1.5 to 2.5 S 0 to 1 <0.5 Fe <1 —

[0049] The hydration process of slag involves the dissolution of theanhydrous compounds, followed by the precipitation of the hydratedcompounds. The agglomerating properties of the slag depend on properactivation.

[0050] Thus, slag is considered a latent hydraulic compound, since itneeds activation to set. The required activation may be provided bythermal energy, chemical activators or through mechanical energy. Theactivation increases the solubility of slag and/or favors theprecipitation of the hydrated compounds.

[0051] The mechanical activation is obtained by milling slag: the higherthe surface area, the larger the dissolution of slag. The particlereduction in size increases surface area, and therefore the contactarea, this increasing the hydration rates.

[0052] Thermal activation is obtained by increasing the temperature ofthe slag, this also increasing the solubility of the slag in solution.

[0053] Chemical activation occurs through the combination of twoparameters: increase in the rate of dissolution of slag caused by anincrease in pH, and anticipation of precipitation of the hydratedcompounds through increase in the concentration of ions in solution orby altering the solubility of the slag compounds.

[0054] The high pH of the slag-containing solution increases the slagsolubility, and consequently its hydration, since the dissolution ofsame is effected through a hydroxyl attack (that is, attack by OH⁻ions), while the dissolution of the cement clinker occurs through ahydrolytic attack (that is, the action of water molecules). Hydroxylions may be provided either by strong bases, such as sodium hydroxide,sodium silicate of low SiO₂/Na₂O ratio or by weak bases, such as sodiumcarbonate, or sodium silicate of high SiO₂/Na₂O ratio.

[0055] The reactivity of the slag depends on various parameters, such asdegree of vitrification, chemical and mineralogical composition andfineness.

[0056] The degree of vitrification dramatically increases reactivity:crystalline slag, obtained by slow cooling, does not hydrate. Thefastest the cooling, the higher the vitrification degree of the slag,and therefore the larger the hydraulic potential of the slag.Experimental data indicate the presence of 3 to 5 weight percent ofcrystals in the slag in order to improve their reactivity.

[0057] The chemical composition of slag is related to the quality of theiron ore, as well as to the nature of the smelter, to the utilization ofcoke or charcoal as fuel, reduction activator, etc. Broadly speaking, abasic slag is more reactive than acidic ones.

[0058] Regarding to the fineness, which may be evaluated by determiningthe surface area, the higher the surface area, the larger thereactivity. Analogously, the higher the temperature, the greater thereactivity of a sample of slag.

[0059] In spite of the fact that several kinds of slag may be useful forthe purposes of the present invention, a preferred slag is a blastfurnace slag, a by-product of the manufacture of blast iron, having avitrification degree higher than 90 weight % and surface area between200 and 600 m²/Kg.

[0060] The rheological properties of the drilling fluid compounded withslag may be modified with the aid of conventional chemical dispersingagents for water-based drilling fluids, such as lignosulfonates,naphthalene sulfonates, phenol sulfonates, phosphates, phosphonates,sulphonated styrene-maleic anhydride, polyacrylate, polymethacrylates,mixtures of polyacrylate and polymethacrylates, acrylic acid-acrylamidecopolymers, or mixtures of any of these classes provided the compoundsare chemically compatible so as to mix while retaining the ability ofdispersing particles in the drilling fluid.

[0061] In view of the fact that slag is a latent hydraulic material,activation is required in order to obtain a slurry of useful features.The activation of slag-containing drilling fluid usually requires theuse of chemical activators besides the thermal activation. In thecompositions taught in the open literature, chemical activators are usedin such amounts as to effect the set in relatively short times, from afew minutes to a few hours.

[0062] As chemical activators, acids and bases may be used, suchcompounds increasing the pH of the liquid portion of a mixture ofwater-based fluid/blast furnace slag. In view of the fact that thethermal energy contributes to the overall activation energy, the kind ofchemical activators and the amount of same may vary. Strong bases areusually employed at low temperatures while weak bases may be used whenthe temperature is higher.

[0063] The activator constituents may be a mixture of bases or basescombined to chelating, complexing or sequestering agents.

[0064] At low temperatures it is advisable to use hydroxides of alkalineand earth alkaline metals such as lithium hydroxide, sodium hydroxide,potassium hydroxide, calcium hydroxide, magnesium hydroxide and bariumhydroxide. The more soluble hydroxides are more adequate activators forlow temperatures. It is believed that the higher solubility in water atlow temperatures is the main reason for the better performance of theseactivators at low temperatures. That is why more commonly usedhydroxides at low temperatures are sodium hydroxide, potassiumhydroxide, lithium hydroxide and barium hydroxide. Those hydroxides havea more pronounced effect on the setting time of the slag-containingcementitious slurry.

[0065] The usual concentrations of such hydroxides are between 0.01 and5 weight % based on the weight of slag in the composition. Preferredconcentrations are between 0.5 and 4 weight % based on the weight ofslag, and the more preferred concentration is between 1 and 2 weight %based on the weight of slag of the composition of cementitious slurry.

[0066] Further, the compositions of the present invention may alsocontain a weaker alkaline metal base as part of the activator agent, forexample, a carbonate or phosphate group-containing weaker base.

[0067] Specifically, the carbonate ion exerts a beneficial effect on thedevelopment of compressive strength and final compressive strength ofthe slag-containing mixture. A few anionic species may partly replacethe carbonate of the activator, this causing a huge effect on thecompressive strength. Alkaline metals and alkaline earth metalcarbonates more commonly applied as a component of the overall activatorcomprise lithium carbonate, sodium carbonate, potassium carbonate,calcium carbonate, magnesium carbonate, barium carbonate, and sodium andpotassium carbonate. Similarly to the alkali-containing hydroxyl groups,higher solubility provides stronger effects. More commonly usedcompounds, employed in the inventive compositions comprise sodiumcarbonate, potassium carbonate and sodium and potassium carbonate. Theconcentrations of those compounds used in the compositions of theinvention comprise 0.01 and 8 weight % based on the weight of slag inthe composition, the preferred composition being between 0.5 and 6weight % based on the weight of slag in the composition, the optimumcontent being between 1.5 and 2.5 weight % based on the weight of slagpresent in the composition.

[0068] Therefore a typical composition of the invention comprises atotal amount of activators between 0.01 weight % and 13 weight % basedon the weight of slag, a strong base such as NaOH comprising between0.01 and 5 weight % based on the weight of slag in the composition,while the weak base such as Na₂CO₃ is present in an amount between 0.01and 8 weight % based on the weight of slag.

[0069] Analogously to the carbonate ion, the phosphate ion is an anionicchemical species, which may contribute to the compressive strength.However, the phosphate ion-containing compounds tend to retard thehydration of slag more strongly than does the carbonate ion. Basicphosphates such as sodium pyrophosphate, sodium hexamethaphosphate andtetrasodium polyphosphate find use in formulations for the control ofsetting time and aid in the development of the compressive strength.Among those, the sodium pyrophosphate shows the best potential. Typicalconcentrations are between 0.01% and 15 weight % based on the weight ofslag, preferably between 0.5 and 8 weight % based on the weight of slag.

[0070] Citrate ion may also contribute to the compressive strength.Alkaline and alkaline earth metal salts of citric acid such as sodiumcitrate, calcium citrate, and potassium citrate are well known.

[0071] As for the rheological properties of the slurry, citrate andphosphate-containing compounds are advantageous when compared tocarbonate or hydroxyl compounds. Citrate and phosphate-containingcompounds are effectively dispersing agents for slag and clays informulations of drilling fluids. Thus, said compounds make possibleslurry compositions of lower water content or of better rheologicalproperties.

[0072] Other components of the activator comprise a chelating,complexing or sequestering agent. Such compounds comprise the phosphateand citrate ion described hereinbefore. Other components are ethylenediamine tetracetic acid (EDTA) and their salts, phosphonic acids andtheir salts, glutamic acid and its salts, etc. It is preferable to usesalts since the acidic forms react with added bases for the chemicalactivation. Soluble salts are preferred, such as sodium and potassiumsalts.

[0073] Concentration of activators is usually selected so as to providemaximum compressive strength for a given concentration in slag. Theoverall optimum concentration is referred based on the weight of slagused in the composition. This is how is selected the optimum ratio ofactivators to slag. The optimum weight ratio of activators to slag isreduced with the increase in temperature. Those optimum ratios may alsovary with the compounds used as activators.

[0074] In the present invention, in the embodiment where thecompositions are incompletely activated, the ratio of activators basedon the slag is chosen so as to incompletely activate the slag so thatthe slag slurries be kept liquid at ambient temperature for prolongedperiods, for example, 72 hours or more. Thus, this ratio is comprised,for example, for sodium hydroxide (NaOH) and sodium carbonate (Na₂CO₃)between 0.01% and 13 weight % based on the weight of slag present in thecomposition, preferably between 1.0 and 10 weight % and more preferablybetween 1.5% and 4.5 weight %.

[0075] As for the embodiment where the composition is kept free of theaddition of activators up to immediately before the composition ispumped into the wellbore, the ratio of activators may be any, providedthe amount of activators make the set after placing the slurry in thewellbore. It is well known, for example, that the excess of activatorsprevents the setting of the slurry.

[0076] In the embodiment of the invention where a portion of therequired amount of activators is added in the amount of 0.01% to 13weight % based on the weight of slag in the composition of slurry, andimmediately before pumping the slurry to the wellbore, another portionof the activators is added, such amount may be any provided the slurryis set within the wellbore.

[0077] For the purposes of the present invention, ambient temperaturemeans that one since which the slurry may be processed up to usualambient temperatures, for example, 25 to 30° C. For countries withambient temperatures higher than 30° C., the amount of activators shouldbe adapted to lower limits, since there will be a certain surfaceactivation due to higher ambient temperatures.

[0078] Contrary to the publications of the literature and in a way notcited nor suggested, which renders the present invention patentablydistinguishable from all published matters, the invention does not makeuse of set inhibitors. (retarders). On its turn, as is thoroughlyexplained in the referred to publications, the use of inhibitorsrequires, in order that the slurry composition be activated, theadditional use of activators to neutralize the effect of inhibitors. Itis evident that such additional chemical reagents and the consequentoperations of mixture and human labor increase the costs of thecementing operation as reported in the state-of-the-art publications.

[0079] On the contrary, in one of the preferred modes of the invention,the slurry in insufficiently activated with chemical activators so thatthe set does not quickly occur at ambient temperature, howeversufficiently activated so that the set quickly occurs within thewellbore.

[0080] In another mode, the slurry is not activated so that the set doesnot occur at ambient temperature, however, chemical activators are addedimmediately before the slurry is pumped into the wellbore, so that theslurry is quickly set at the temperature of the wellbore. This featuremakes possible that the slurry may be prepared in advance, improving thequality as a result of the composition being uniform and also reducingthe time of operation, since it is only necessary to displace the slurryto the wellbore.

[0081] For two formulations of the inventive slurries having optimizedslag ratios and activators, the more dispersed formulation will developas a whole better compressive strength after setting. Severalcomplexing, sequestering or chelating agents mentioned above are gooddispersing agents for slag containing drilling fluids.

[0082] Generally, in the laboratory the storable compositions of theinvention may be prepared by mixing blast furnace slag, the additives inthe right amount and the aqueous continuous phase The order of additionof the constituents is not critical, however it is preferred to addfirst the water based fluids, followed by cementing and drillingadditives, activators and slag.

[0083] The mixture is prepared in an agitator with controlled rotatingspeed such as a Waring blender. Constituents are weighed in a scalehaving a precision of 0.01 g. After weighing, the water-based fluids(water based drilling fluid, fresh water, brine, etc) are added to thecup of the blender and agitated under a rotation of 4,000 rpm, duringaround 15 seconds. The solid constituents are added over that liquidmixture one after the other, under a rotation of 4,000 rpm, for a periodof time sufficient so that all constituents are dispersed or dissolve inthe slurry being prepared. During the addition of slag, at the end ofthe mixture, the rotation is generally increased to 12,000 rpm in orderto improve and facilitate its dispersion in the slurry. Mixing time maytake from a few seconds to a few minutes. Generally, additives andactivators are agitated for around 50 seconds, while the slag may beagitated for a few minutes.

[0084] In the laboratory, the assessment of the properties of the slurryis effected through tests such as compressive strength, thickening time,rheology, filtrate (fluid loss), density and free water. Those tests areeffected according to SPECIFICATIONS 10 of the American PetroleumInstitute (API).

[0085] Before storing the slurry for assessing the storage time, theproperties of the composition are measured so as to ascertain itssuitability to oil and gas wells. Once the suitability of the slurry isassessed, the slurry is prepared, the rheological parameters (plasticviscosity, yield point and gels) are measured and then the slurry isstored in capped flasks in order to avoid loss of water. The rheologicalparameters are periodically measured at 27° C. in order to monitor howthe properties of the slurries have been modified. Whenever the slurryis too viscous and gelled, it is difficult to pump and therefore itsproperties are no longer suitable, the set is reached in a few hours. Onstating the hardening of the slurry (set), the test is put to an end andthe storage time is counted as the number of days between thepreparation of the slurry and the day before the setting.

[0086] In the field, the storable compositions of the invention areprepared in a mixing tank of the rig or of the cementing unit (this is atank provided with mechanical agitators and/or hydraulic recirculation).The constituents are weighed in a scale of 1-kg precision. Afterweighing, the water-based fluids (water based drilling fluid, plainwater, brine, etc.) are added to the mixing tank and agitated for a fewminutes. The solid constituents are added on that liquid mixture oneafter the other, in a period of time such that such constituentsdisperse or dissolve in the slurry, which is being prepared. Normally afew hours are needed to prepare a slurry.

[0087] The present invention will now be illustrated by the followingexamples, which should not be construed as limiting same.

EXAMPLE I

[0088] During the cementing operation of the casing having 9⅝″ diameter(0.244 m) of a well from the Parana basin, county of Pitangas, Brazil, asample of the slurries prepared in the cementing unit was collectedbefore being displaced to the well. Two slurries were used in the well.The first, called extended, was placed over the second slurry, callednormal, which was placed near the shoe of the casing.

[0089] TABLE 2 shows the composition of the extended slurry used in thewell under study. A sample of such slurry was stored for 3 days atambient temperature (around 24° C.) and not setting (solidification)occurred. Immediately after the collection of the sample the compressivestrength was determined, which after curing for 24 hours at 55° C. (131°F., static temperature of the well), was 1220 psi (8.412 MPa). Theobtained value is suitable for cementing of casing. The CBL/VDL/CETlogging of the well indicates that the slurry has fulfilled the goals ofthe cementing operation. TABLE 2 Constituent Concentration H₂O 74.2% byvolume Starch 1.25 lb/bbl (3.57 Kg/m³) Active Clay 2.4 weight % based onthe weight of slag NaOH 1.5 weight % based on the weight of slag Na₂CO₃2.5 weight % based on the weight of slag Blast furnace slag 250 lb/bbl(713 Kg/m³)

[0090] TABLE 3 below shows the composition of the normal slurry used inthe well under study. A sample of said slurry was stored for 16 days atambient temperature, around 24° C., and no solidification occurred. Thecompressive strength at 8 hours curing at a temperature of 84° C. (184°F., bottom hole static temperature) measured 3 and 16 days afterpreparing the slurry, was respectively 1017 psi (7.012 Mpa) and 1000 psi(6.895 Mpa). Those values are suitable for the purposes of the casingcementing.

[0091] TABLE 4 below shows that the rheological parameters of the slurryundergo minor changes with storage time. The cement logging of the wellindicated a good cementing quality. TABLE 3 Constituent ConcentrationLow solids Drilling fluid 32% by volume H₂O 33% by volume Fluid LossAdditive 0.1 weight % based on the weight of slag NaOH 1.2 weight %based on the weight of slag Na₂CO₃ 2.0 weight % based on the weight ofslag Dispersant 1.5 lb/bbl (4.28 Kg/m³) Blast furnace slag 350 lb/bbl(999 Kg/m³)

[0092] TABLE 4 Rheology @ 27° C. (80° F.) 5 days 6 days 10 days PlasticViscosity 1.12 P 0.94 P 0.91 P Yield Point 4 7 lbf/100 ft² 7 lbf/100 ft²(1.9 Pa) (3.4 Pa) (3.4 Pa) Gel 10 s 3 lbf/100 ft² 3 lbf/100 ft² 6lbf/100 ft² (1.4 Pa) (1.4 Pa) (2.9 Pa) Gel 10 min 20 lbf/100 ft² 18lbf/100 ft² 30 lbf/100 ft² (9.6 Pa) (8.6 Pa) (14.4 Pa)

[0093] A sample of the slurry was put aside immediately after beingprepared, before the displacement to a well in the Northeast ProductionArea, state of Rio Grande do Norte, Brazil. TABLE 5 below shows thecomposition of the slurry used in the cementing of the casing of theproduction well under study. The sample was stored for 24 days atambient temperature (around 24° C.) in the absence of solidification.The compressive strength values in 24 hours curing time at 73° C. (164°F., bottom hole static temperature) as measured immediately after theslurry was prepared and 6 days after, were respectively 1900 psi (13.10MPa) and 1810 psi (12.84 Mpa).

[0094] TABLE 6 below shows further properties of said slurry. Thosevalues are suitable for cementing the production casing.

[0095] TABLE 7 below shows that the rheological parameters of the slurryundergo only slight changes with the storage time. The cement logging ofthe well indicated a suitable cementing performance. TABLE 5 ConstituentConcentration Drilling fluid 42.5% by volume Industrial water 20.8% byvolume Fluid Loss Additive 3.66 lb/bbl (10.4 Kg/m³) NaOH 1.2 weight %based on the weight of slag Na₂CO₃ 3.1 weight % based on the weight ofslag Dispersant 1.1 lb/bbl (3.1 Kg/m³) Blast Furnace Slag 356 lb/bbl(1016 Kg/m³)

[0096] TABLE 6 Test Temperature Property (° C.) Value read Densitylb/gal 20 14.8 (Kg/m³) (1.773) ( Fluid Loss cm³/30 min 51 26 m³/s (1.4 ×10⁻⁸) Plastic Viscosity. Pa 27 1.39 51 0.81 YieldPoint. lbf/100 ft² 2741 (19.6) (Pa) 51 12 (5.7) Initial Gel lbf/100 ft² 27 27 (Pa) (12.9) 518 (3.8) FinalGel lbf/100 ft² 27 38 (Pa) (18.2) 51 20 (9.6)Freewater.cm³/2 h 27 Zero (m³) (zero)

[0097] TABLE 7 Rheology @ 27° C. (80° F.) Start 3 days 8 days 23 daysPlastic Viscosity (Pa) 1.39 1.54 1.66 1.71 Yield Point lbf/100 ft² 41 3744 92 (Pa) 19.6 17.7 21.1 44.0 Gel 10s lbf/100 ft² 27 24 25 87 (Pa) 12.911.5 12.0 41.7 Gel 10 min lbf/100 ft² 38 37 46 105 (Pa) 18.2 17.7 22.050.3

FURTHER EXAMPLES

[0098] TABLE 8 below shows formulations A up to H which are examples ofslag slurry which remained liquid at ambient temperature for at least 7days, and solidify in a few hours at the temperature of the well. TABLE8 A B C D E F G H Formulation Drilling Fluid 0.607 0.403 0.403 0.4030.403 0.35 0.354 0.324 (% by volume) Water (% by volume) — 0.283 0.2830.283 0.283 0.35 0.354 0.324 NaOH (weight % based on the 1 1 1 1 1 2 4 0weight of slag) Na₂CO₃ (weight % based on the 1.5 2 2 2 2 6 6 1.5 weightof slag) Dispersant lb/bbl 2.5 2 2 2 2 1 1 2 (Kg/m³) 7.1 5.7 5.7 5.7 5.72.9 2.9 5.7 2 Fluid loss additive (weight % — 0.78 0.78 0.78 0.78 — —1.2 based on the weight of slag) Temperature stabilizer — 1.17 1.17 1.171.17 — — — (weight % based on the weight of slag) Slag lb/bbl 400 300300 300 300 300 300 350 (Kg/m³) 1.141 856 856 856 856 856 856 999Surface area (m²/Kg) 400 400 582 582 235 400 236 400 PropertiesCompressive psi 960 2,630 1,782 1,782 2,371 1,160 1,282 1,160 Strength¹(Mpa) 6.62 18.13 12.29 12.29 16.35 8.00 8.84 8.00 Curing Temperature (°C.) 80 150 150 150 150 80 120 98 Storage Time (days) 20 >50 >59 31 >1047 7 >27 Storage Temperature (° C.) 22 22 22 35 22 22 22 22

[0099] Formulation of TABLE 9 is formulation F of TABLE 8 without theaddition of activators NaOH and Na₂ CO₃. Formulation I was kept fluidfor more than 67 days. By adding activators after 28 days, the slurrysubjected to the curing temperatures of formulation F, 80° C., showedthe highest compressive strength, 1,626 psi (11.21 MPa), which issuitable for cementing oil wells. TABLE 9 I Formulation Drilling fluid(% by volume) 35 Water(% by volume) 35 NaOH (weight % based on theweight of slag) — Na₂CO₃ (weight % based on the weight of slag) —Dispersant lb/bbl 1.0 (Kg/m³) 2.9 Slag lb/bb/ 300 (Kg(m³) 856 Surfacearea (m²/Kg) 400 Properties Compressive strength¹ psi *1.626 (Mpa)*11.61 Curing Temperature (° C.) 80 Storage Time (days) >67 StorageTemperature (° C.) 22

We claim:
 1. Storable compositions for cementing oil and gas wellscontaining high-furnace slag as hydraulic material, activators,additives and an aqueous continuous phase forming a cementitious slurry,wherein the amount of activators in said compositions based on theamount of slag is insufficient to effect the setting of the compositionfor at least 72 hours.
 2. Storable compositions according to claim 1,which comprise: Of from 200 to 500 lb (570 to 1428 kg/m³) ofblast-furnace slag per each bbl of slurry to be prepared; Of from 0.01weight % to 13 weight % of activators, based on the weight of said slag;The balance of the composition being made up of an water-based drillingfluid, and additives.
 3. Storable compositions for cementing oil and gaswells containing blast-furnace slag as hydraulic material, activators,additives and an aqueous continuous phase forming a cementitious slurry,wherein the amount of activators is added immediately before pumping thecomposition to said well, in any amount which is efficient to effect thesetting.
 4. Storable compositions according to claim 1, which comprise:Of from 200 to 500 lb (570 to 1428 kg/m³) of blast-furnace slag per bblof slurry to be prepared; Any weight amount of activators, based on theweight of said slag, said amount being that which is required to effectthe setting and being added immediately before pumping said compositionto the well; The balance of the composition being made up of anwater-based drilling fluid, and additives.
 5. Storable compositions forcementing oil and gas wells containing blast-furnace slag as hydraulicmaterial, activators, additives and an aqueous continuous phase forminga cementitious slurry, wherein a portion of the activators is addedwhile said composition is being prepared and the remaining requiredportion is added immediately before the composition is pumped into thewell.
 6. Storable compositions according to claim 5, which comprise: Offrom 200 to 500 lb (570 to 1428 kg/m³) of high-furnace slag per bbl ofslurry to be prepared; Of from 0.01 weight % to 13 weight % ofactivators based on the weight of said slag, said additives being addedwhile said composition is being prepared; and The remaining of theactivators being added immediately before pumping the composition to thewell in any amount, based on the weight of slag, said amount being thatone required to effect the setting of the composition; The balance ofthe composition being made up of water-based fluid, and additives. 7.Storable compositions according to claim 1, 3 or 5, wherein the settingof said compositions is obtained by combining the chemical action of theactivators and the thermal action of the temperature of the well bottom.8. Storable compositions according to claim 1, 3 or 5, wherein saidcompositions are free of set inhibitors.
 9. Storable compositionsaccording to claim 1, 3 or 5, wherein the blast-furnace slag is aby-product from the melting of iron ores.
 10. Storable compositionsaccording to claim 1, 3 or 5, wherein the activator comprises one ormore alkali compounds, isolated or in admixture, combined or not withchelating, complexing or sequestering agents.
 11. Storable compositionsaccording to claim 2 or 6, wherein the preferred amount of activator isbetween 0.1 to 10 weight %, and the most preferred amount is between 1.5and 4.5 weight % based on the weight of slag of the composition of saidcementitious slurry.
 12. Storable compositions according to claim 1, 3or 5, wherein the activator contains a strong alkali bearing an hydroxylion.
 13. Storable compositions according to claim 12, wherein the alkaliis a sodium hydroxide, potassium hydroxide or a mixture of same, saidalkali being present in an amount of 0.01 to 5 weight % based on theamount of slag in the composition, preferably between 0.5 and 4 weight%, and still more preferably between 1 and 2 weight % based on theamount of slag in the cementitious slurry. 14 Storable compositionsaccording to claim 1, 3 or 5, wherein the activator contains a weakalkali. 15 Storable compositions according to claim 14, wherein the weakalkali comprises a carbonate ion.
 16. Storable compositions according toclaim 15, wherein the carbonate ion is a sodium carbonate, a potassiumcarbonate or a mixture of same, in an amount of 0.01 to 8 weight % basedon the amount of slag in the composition, preferably between 0.5 and 2.5weight %, and still more preferably between 1.5 and 2.5 weight % basedon the weight of slag in the cementitious slurry.
 17. Storablecompositions according to claim 14, wherein the weak alkali comprises aphosphate ion.
 18. Storable compositions according to claim 17, whereinthe phosphate ion is a basic phosphate such sodium pyrophosphate, sodiumhexamethaphosphate or tetrasodium polyphosphate, in an amount between0.01 and 15 weight % based on the weight of slag, more preferablybetween 0.5 and 8.0 weight %.
 19. Storable compositions according toclaim 10, wherein the chelating, sequestering or complexing agentscomprise phosphates, citrates, ethylene diamine tetracetic acid and itssalts, phosphonic acids and their salts, glutamic acid and its salts inan amount between 0.01% and 15 weight % based on the weight of slag,more preferably between 0.5% and 8%.
 20. Storable compositions accordingto claim 2, 4 or 6, wherein the water based fluid comprises a fluidwhere the continuous phase is water.
 21. Storable compositions accordingto claim 20, wherein the water continuous phase comprises ordinary freshwater, brine, sea water, isolated or in admixture in any amount, or anyother water based drilling fluids.
 22. Cementitious slurries useful forcementing oil and gas wells the composition of which is according toclaim 2, 4 or 6, wherein the slurries are prepared by admixing theconstituents in the required amounts at ambient temperature in a tankprovided with mechanical and/or hydraulic circulation agitators (mixingtank), the solid constituents being added on the mixture of water basedfluids, one at a time, during a sufficient time so that said solids aredispersed or dissolved in the liquid phase, the slurry being kept fluidfor at least 72 hours.
 23. Use of the storable compositions according toclaim 2, 4 or 6 in the cementing of oil and gas wells.